An inverter of NPC type is known in the prior art. It comprises in particular three parallel switching arms connected between a positive line and a negative line of a DC bus. Each switching arm is fitted with four power switches linked in series, the power switches being controlled by a control module for the variable speed drive based on a control voltage vector with the aim of controlling an electrical load. The variable drive also comprises a mid-point realized between two capacitors connected in series between the positive line and the negative line of the DC bus, a current passing through the said mid-point.
In the control of an NPC inverter, the main difficulty resides in the regulation of the electrical potential of the mid-point. Indeed, the electrical potential of the mid-point is obtained by dividing the voltages across the terminals of the two capacitors and therefore varies according to the quantity of current delivered to the load. Now, if the potential of the mid-point varies a great deal, a voltage excess appears across the terminals of the capacitors, and this may cause instabilities or even an impairment of these capacitors.
U.S. Pat. No. 6,795,323, U.S. Pat. No. 6,490,185 or U.S. Pat. No. 7,495,938 propose various schemes for regulating the electrical potential of the mid-point. However, these schemes are not suitable when the inverter has to operate at full voltage, or stated otherwise in overmodulation.